Thin-disk lasers and amplifiers include a solid-state gain-medium in the form of a thin disk. Typically such a thin-disk gain-medium has a diameter between about 5 millimeters (mm) and 50 mm and a thickness between about 50 micrometers (μm) and 500 μm. The thin-disk gain-medium has a reflective coating on one surface thereof and is surface-cooled by placing that reflective-coated surface in thermal contact with a heat-sink, which may be actively cooled. Cooling is very effective because of a high surface to volume ratio for the thin-disk. Further, because the surface area of the thin-disk is very much greater than the thickness, heat flow from the thin-disk into the heat sink is essentially unidirectional and in the thickness direction of the thin-disk. This minimizes thermal-lens effects in the gain-medium.
One problem with a thin-disk gain-medium is providing adequate absorption of optical pump radiation. The gain-medium in a thin-disk form can be somewhat more heavily doped than would be a solid-state gain-medium in conventional rod or bar form, but because of the very thin form of the disk, typically no more than about 10% of a pump radiation beam can be absorbed in a forward and reverse pass through the disk via the surface thereof opposite the reflective-coated surface.
Several prior-art schemes having varying degrees of effectiveness and complexity have been proposed for causing a beam of pump radiation to make two or more forward and reverse passes through a thin-disk gain-medium. Many of these, however, do not pay particular attention to the form of the intensity distribution created by the combination of double passes on the disk. Ideally, i.e., for optimum quality of an output laser beam or an amplified signal beam, the intensity distribution of the pump radiation should approximate a radially symmetric Gaussian or Super-Gaussian distribution.
Perhaps the most elegant of prior-art arrangements for multi-pass pumping of a thin-disk gain-medium is an arrangement described in U.S. Pat. No. 6,577,666. This patent was granted to a group of researchers at Universität Stuttgart Institut für Strahlwerkzeuge, where extensive, internationally-recognized development work has been carried out on high-power, thin-disk lasers and amplifiers. This arrangement has been enthusiastically adopted by several manufacturers and developers of thin-disk lasers.
In the arrangement of the '666 patent, a plurality of retro reflecting mirror pairs is used in combination with a parabolic mirror to repeatedly re-image a thin-disk gain-medium, illuminated by the pump beam, back on itself. The prisms are arranged to translate the beam between each imaging incidence on the thin-disk such that the beams progress radially around the parabolic mirror between incidences on the thin-disk. This causes the plurality of re-imaging passes to be distributed in a more or less conical form to achieve a desired radial symmetry of the combined pump-radiation on the thin-disk. In U.S. Pat. No. 6,778,580, the same group also proposes using a similar arrangement for multi-pass amplification of small signals in a thin-disk amplifier.
What appears to have been overlooked in the above-referenced developments, however, is the issue of polarization-plane maintenance of both pump-radiation and signal-radiation. In the above discussed arrangements, as the pump-radiation or signal radiation is successively re-imaged on the thin-disk the image rotates with each successive incidence on the thin-disk. If the radiation is plane-polarized, the polarization-plane also rotates
Polarization maintenance can be very important for reasons as follows. Several common sources of optical pump radiation deliver radiation which is plane-polarized. Such sources include single emitter diode-lasers and diode-laser bars including a plurality of emitters. If the radiation is not transmitted through a common optical fiber or bundle, it will stay plane-polarized. Many of the gain-media favored for use in thin-disk form have polarization-dependent absorption at strong absorption peaks or polarization-dependent gain at strong emission wavelengths.
By way of example, single crystal neodymium-doped yttrium vanadate (Nd:YVO4) has a peak absorption at a wavelength of 808 nanometers (nm) and a strong emission line at a wavelength of 1064 nm. The absorption and emission cross-sections are strongest, and significantly so, for radiation plane-polarized parallel to the c-axis of the crystal. If polarization of pump radiation rotates during multi-pass pumping on the Nd:YVO4, the effective absorption of radiation for a given number of incidences on the disk will be reduced from the peak obtainable value.
Similarly, in multi-pass amplification, rotation of the plane of polarization of the signal will reduce the available gain available for a given number of incidences on the thin-disk. Further, the amplifier output will be de-polarized, at least partially. In certain cases where the amplifier output is used directly for an application a depolarized output beam may not present a problem. If, however, the amplifier output is to be frequency-doubled by type-1 frequency-doubling, a plane-polarized output beam is necessary for maximum conversion (doubling) efficiency.
There is a need for a multi-pass imaging arrangement that can preserve the polarization plane of pump-radiation or signal-radiation on a think-disk gain-medium. Preferably the arrangement should retain most if not all of the advantages of the more successful prior-art multi-pass imaging arrangements.